This invention relates to a control system for controlling the condenser pressure in a refrigeration system of the type having an air-cooled condenser. While the invention may be employed in a variety of such refrigeration systems, it is particularly useful in an all-weather air-conditioning system which operates at different system capacities and in the presence of a broad range of outside ambient temperatures. Accordingly, the invention will be described in that environment.
The condenser of an air-conditioning system is usually located out-of-doors or in heat exchange relation with outdoor air and is therefore subjected to widely varying ambient temperatures. If the system operates during cold weather, the outdoor temperatures may drop sufficiently low to materially reduce the condensing temperature of the refrigerant in the condenser. This produces a corresponding reduction in head pressure on the high pressure side of the refrigeration system, resulting in a decreased pressure differential across the expansion valve or other refrigerant metering device in the system. Because of the reduced pressure difference across the expansion valve, the flow rate decreases and less refrigerant flows from the condenser to the evaporator. As a consequence, the evaporator is starved and the heat load placed on the evaporator may not be satisfied.
If adequate head pressure is not maintained at low ambient temperatures, the evaporator temperature may drop below freezing, allowing condensed moisture to freeze on the evaporator. As the layer of ice builds up on the evaporator, the evaporator becomes insulated from the refrigeration load and a further reduction in effectiveness occurs.
Conversely, if the condenser head pressure is permitted to rise too high, higher than necessary operating costs are incurred, the expansion valve performs poorly, and in some cases nuisance trip-outs on high pressure occur.
Control systems have been developed for holding the condenser head pressure reasonably constant under varying ambient temperatures. In one such system, the condenser pressure is held at a fixed level, or control point, by keying the speed of a variable speed fan for the condenser to the condensing temperature, and thus to the condenser pressure. As the condenser pressure tends to deviate from the desired control point, the fan speed varies accordingly. For example, as it tends to decrease in response to a falling outside ambient temperature, the fan speed is automatically reduced. The volume of air blown across the condenser therefore decreases and this limits the amount of heat that can be extracted from the refrigerant as it flows through the condenser, insuring that the refrigerant pressure remains relatively close to the control point and does not fall below the minimum necessary for proper operation of the refrigeration system. By holding the pressure on the high side of the system at the control point, the pressure difference across the expansion or metering device will be sufficient to properly feed the evaporator and satisfy the head load.
In another well-known head pressure control system, useful when several fans are employed to cool the condenser, a fixed condenser pressure at a desired control point is obtained, in the presence of cooling air temperature changes, by cycling the fans on and off as necessary. The lower the ambient temperature, the smaller the number of fans needed.
Unfortunately, the prior control systems are capable of controlling at only a single condenser pressure and are therefore set at the head pressure required to maintain adequate refrigerant flow to the evaporator at full capacity. If the refrigeration system is of the type that has a reduced capacity operating mode, wherein the refrigerant flow rate is purposely reduced when the heat load drops, the previously developed control systems will maintain the condenser pressure at the same relatively high control point at both full and reduced capacity. During reduced capacity operation, the condenser pressure will therefore be substantially above that which is needed to adequately feed the evaporator. This higher than necessary head pressure results in unnecessary and wasteful power consumption. It also requires the expansion valve to throttle down further, causing unnecessarily high liquid refrigerant velocities. The high condenser pressure also results in greater changes in liquid flow for a change in valve position, making the expansion valve control loop more sensitive and unstable.
The present invention, on the other hand, changes the control point as the operating mode changes. During full capacity operation, the condenser pressure is maintained constant at a relatively high control point, and during reduced capacity operation the head pressure is held fixed at a much lower control point. This results in optimum operation at all outdoor air temperatures and at both full and reduced capacity, thereby maximizing the efficiency and minimizing the power requirements and operating costs.